A Category 4 storm has winds from 131 mph to 155 mph (210 kph to 248 kph) and storm surges from 13 feet to 18 feet (4.3 meters to 6 meters), based on the Saffir-Simpson scale that measures hurricane intensity.

In New Orleans, Mayor Ray Nagin postponed a planned return of residents to the city and ordered those who had returned to leave because of the threat posed by Rita.

The commander of the Army Corps of Engineers told CNN that only 40 percent of the pumping stations in New Orleans were working.

"We think something on the line of 3 inches over six hours would probably put 2 to 4 feet of water in the lower-lying sections of the city," said Lt. Gen. Carl Strock. "... The levees also are a concern. They would not stand any kind of significant storm surge."

Coast Guard Vice Adm. Thad Allen, the federal point man for recovery efforts, said 500 buses were available to take people out of the city, and it was possible commercial jets also would be used to evacuate people.

Louisiana Gov. Kathleen Blanco urged residents of southwestern Louisiana to be ready to evacuate on short notice.

Because Rita could hit Texas, which already has taken in tens of thousands of Louisiana residents after Katrina, she urged them to go north instead of west. (Full story)

Tuesday night, Blanco sent a letter to President Bush asking him to declare a federal state of emergency.

Rita was moving west and had slowed to about 13 mph (20 kph). Its hurricane-force winds extended out 45 miles (75 kilometers) and tropical storm-force winds extended 140 miles (220 kilometers) from its center into south Florida.

Key West Mayor Jim Weekley said about half of the city's residents left ahead of the storm, leaving about 13,000 people in the city.

He said people took the evacuation order more seriously in the wake of Hurricane Katrina, adding that only about 25 percent of the city's residents left in previous hurricanes.

Mandatory evacuation orders were issued Monday for the Keys, but many residents insisted on staying. The evacuation was the fifth ordered for the Keys in the past two years.

"In the Keys, a lot of people are stubborn," said Craig Fugate, director of the Florida Division of Emergency Management.

CNN's Dan Lothian reported from Key Largo that he saw some people still putting up boards on their windows as late as Tuesday morning.

Gov. Bush said more than 1,600 residents of Monroe County, which includes all the Keys, were in shelters elsewhere in Florida. All three hospitals in the county were evacuated, as well as its nursing homes.

A hurricane warning stretched from just south of Florida City in southeast Florida down to East Cape Sable, around the tip of the peninsula and then north to Chokoloskee. All of the Florida Keys are included in the warning.

Hurricane warnings were still in effect for three Cuban provinces, including Havana. CNN's Lucia Newman said more than 58,000 Cubans were evacuated from coastal areas.